1. Field of the Invention
The present invention relates to an improvement in a method of making a semiconductor device comprising a compound semiconductor of arsenic.
More specifically, the present invention relates to a method of forming a thermal oxidation film containing arsenic in the manufacture of a semiconductor device comprising a compound semiconductor of arsenic.
2. Description of the Prior Art
Recently, semiconductor devices containing a semiconductor compound, for example, gallium arsenide have been studied because they have satisfactory high-frequency characteristics. However, since they principally consist of compound semiconductors instead of a semiconductor of single element of the conventional semiconductor device, when a heat treatment is applied thereto for forming a thermal oxidation layer or for annealing an oxidation layer to stabilization, several problems take place, for which the compound semiconductor devices require further development.
In general, various heat treatments are carried out in making semiconductor devices, for example, for forming a thermal oxidation film, annealing the gate oxide film, forming an isolation film by selective oxidation, and so on.
Conventional ways of conducting the heat treatment of the compound semiconductor containing arsenic are elucidated hereafter.
For forming the thermal oxidation film of the compound semiconductor, a substrate of the compound semiconductor, for example, gallium arsenide is disposed in an open tube, and the substrate is heat-treated at a temperature of 500.degree. C.-600.degree. C. by a flow of the reaction gas consisting of air, oxygen, or steam. Annealing for stabilization of the gate oxide film of compound semiconductor is made by exposing the oxide film, which is formed by an anodic oxidation method, to a flow of a reaction gas of one or a mixture of hydrogen, nitrogen and oxygen, at a temperature of above 300.degree. C. in an open tube.
However, the conventional arsenic-containing compound semiconductor devices made by the conventional heat treatment process have drawbacks, for example, such that the gate oxide film of MOS FETs have low specific resistivities hence have a poor breakdown voltage and leakage current for actual use.
When an anodic oxide film, which is known as being superior in electric characteristics to the thermal oxide film, is used for the isolation region in order to obtain a flat finished face, there is a problem that the anodic oxide film is chemically unstable and hence is likely to be damaged by a chemical solution such as an etchant, and accordingly, it has been difficult to manufacture a semiconductor integrated circuit comprising the arsenic-containing compound semiconductor.
The inventors herein have made various studies of the conventional thermal oxide films, and found the following conclusions:
(1) In the conventional heat treatment for forming a thermal oxide film, oxides mainly of gallium sesqui-oxide (Ga.sub.2 O.sub.3) and arsenic tri-oxide are formed on the principal face of the substrate. However, since the vapor pressure of the arsenic tri-oxide is more than 1 atmosphere at the abovementioned temperature of the heat treatment, a considerable portion of the arsenic tri-oxide in said thermal oxide film evaporates, thereby resulting in the formation of an oxide film consisting principally of gallium sesqui-oxide and having a relatively low specific resistivity. PA1 (2) Also, in the heat treatment for annealing a gate oxide film or for forming a selective oxidation film, the arsenic trioxide is likely to evaporate, thereby changing the composition into that consisting principally of gallium sesqui-oxide and having a relatively low specific resistance.